Capacitorless DRAM on bulk silicon

ABSTRACT

A method of forming capacitorless DRAM over localized silicon-on-insulator comprises the following steps: A silicon substrate is provided, and an array of silicon studs is defined within the silicon substrate. An insulator layer is defined atop at least a portion of the silicon substrate, and between the silicon studs. A silicon-over-insulator layer is defined surrounding the silicon studs atop the insulator layer, and a capacitorless DRAM is formed within and above the silicon-over-insulator layer.

FIELD OF THE INVENTION

This invention relates generally to a localized silicon-on-insulator(“SOI”) semiconductor design, and, more particularly, to the creation oflocalized oxide in the array of dynamic random access memories (“DRAM”).

BACKGROUND OF THE INVENTION

Use of a silicon-on-insulator, or SOI, substrate generally enables themanufacture of typical circuit elements over an insulator, such asoxide. In one application, capacitorless DRAMs may be formed on SOI. Useof the SOI design versus a traditional silicon substrate increases thefloating body effect for the access transistors of these capacitorlessDRAMs, yielding far more effective storage. The programming of thefloating bodies in such DRAMs may be done either by impact ionization(“II”) or by gate induced drain leakage (“GIDL”). The sensing isnon-destructive and is done using a resistance or current sensing methodat a lower voltage. Further description of capacitorless DRAM via GIDLmay be found in Yoshida et al., A Design of a Capacitorless 1T-DRAM CellUsing Gate-induced Drain Leakage (GIDL) Current for Low-power andHigh-speed Embedded Memory, Technical Digest—International ElectronDevices Meeting 2003, pp. 913-916 (IEEE Cat. No. 03CH37457, 2003), thecontents of which are incorporated herein in its entirety.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a method of formingcapacitorless DRAM over localized silicon-on-insulator is disclosed. Themethod comprises the following steps: A silicon substrate is provided,and an array of silicon studs is defined within the silicon substrate.An insulator layer is defined atop at least a portion of the siliconsubstrate, and between the silicon studs. A silicon-over-insulator layeris defined surrounding the silicon studs atop the insulator layer, and acapacitorless DRAM is formed within and above the silicon-over-insulatorlayer.

According to another embodiment of the invention, a method of forming amemory chip is disclosed. The method comprises the following steps: Aperiphery region and a memory array region are defined on the memorychip. At least one silicon-over-insulator region is formed in the memoryarray region, without forming a silicon-over-insulator region in theperiphery region. At least one capacitorless DRAM is formed on andwithin the at least one silicon-over-insulator region.

According to another embodiment of the invention, a memory device isdisclosed. The memory device comprises a source and a drain. The memorydevice further comprises a floating body formed between the source andthe drain, the floating body defined within a localizedsilicon-over-insulator. The memory device further comprises a gateadjacent the floating body.

According to another embodiment of the invention, an integrated circuitis disclosed. The integrated circuit comprises a periphery region, andan array region. At least one localized silicon-over-insulator is formedwithin the array region. The integrated circuit further comprises asource and a drain formed within the array region. A floating body isformed between the source and the drain within the at least onelocalized silicon-over-insulator. The integrated circuit furthercomprises a gate adjacent the floating body.

According to another embodiment of the invention, a system is disclosed.The system comprises a source, and a first drain and a second drain. Thesystem further comprises a first floating body formed between the sourceand the first drain, and a second floating body formed between thesource and the second drain, the floating bodies defined within alocalized silicon-over-insulator. The system further comprises a firstgate adjacent the first floating body, and a second gate adjacent thesecond floating body.

According to one embodiment of the invention, a method of operating acapacitorless DRAM is disclosed. The method comprises the followingsteps: A floating body is placed in a first state, and the first stateis detected by measuring a first current at a source of thecapacitorless DRAM. The floating body is defined within a localizedsilicon-over-insulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a portion of a memorydevice on which a first step of a process for forming localized SOIaccording to one embodiment of the present invention has been performed.

FIG. 2 illustrates the memory device of FIG. 1 on which a second step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 3 illustrates the memory device of FIG. 1 on which a third step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 4 illustrates the memory device of FIG. 1 on which a fourth step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 5 illustrates the memory device of FIG. 1 on which a fifth step ofa process for forming localized SOI according to one embodiment of thepresent invention has been performed.

FIG. 6 illustrates a capacitorless DRAM built over the localized SOIsubstrate of FIG. 5.

FIG. 7 is a partial top-down plan view of the capacitorless DRAM of FIG.6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the preferred embodiments of the present invention illustratelocalized SOI in combination with capacitorless DRAM, it should beunderstood that these methods of forming localized SOI may beincorporated into the fabrication of other integrated circuits as well.In addition, while the following methods are described in terms ofparticular DRAM fabrication techniques, as would be well known to thoseof skill in the art, such techniques may be replaced by other methods offabricating and modifying semiconductor materials.

SOI is typically created by a uniform layer transfer. Thus, in order tomanufacture capacitorless DRAM over SOI, for example, the entire surfaceof the array and periphery of the memory device incorporates a SOIsubstrate. However, while the SOI is desirable within the array, wherethe insulator enhances floating body effects, it adversely affectsperformance of the chip in the periphery.

Moreover, the creation of SOI via layer transfer is a difficult,time-consuming and expensive process. The fusion of different siliconand insulative layers poses many technical problems and must beperformed at particular temperatures under particular conditions.

Therefore, there is a need in the art to create localized areas of SOIby conventional semiconductor fabrication techniques while leaving therest of the chip unmodified. There is a further need for a method ofmaking capacitorless DRAM using bulk silicon by conventional DRAMfabrication techniques. Thus, the advantages of capacitorless DRAM mightbe achieved without the expense and difficulty of creating SOI via layertransfer.

FIGS. 1 through 5 illustrate one method for forming localized SOIinexpensively and efficiently. According to this method, the SOIstructure(s) may be formed solely within the array of a memory chip,leaving the periphery to be formed on and within a typical semiconductorsubstrate.

Although not shown in any figure, the following method of forminglocalized SOI may be performed using any typical substrate 10, oftenformed from a silicon wafer. In other embodiments, the substrate 10 maycomprise other suitable materials (e.g., other group III-IV materials),or epitaxial layers formed over single crystal wafers.

With reference initially to FIG. 1, a thin, thermally-grown dielectriclayer (not shown), comprising pad oxide in a preferred embodiment, ispreferably first formed over the substrate 10. A hard mask layer 12,such as silicon nitride, may then be deposited over the substrate 10 anddielectric layer. The hard mask layer 12 may be formed by any well-knowndeposition process, such as sputtering, chemical vapor deposition (CVD)or spin-on deposition, among others. Although the hard mask layer 12comprises silicon nitride in a preferred embodiment, it must beunderstood that it may also be formed from silicon oxide, for example,or other materials that can protect underlying substrate during asubstrate etch and also withstand additional processing, as will beapparent from the fabrication steps described further below.

In a step also not illustrated in the figures, the hard mask layer 12may then be patterned using a photoresist layer formed over the hardmask layer 12. The photoresist layer may be patterned to form a maskusing conventional photolithographic techniques, and the hard mask layer12 may be anisotropically etched through the patterned photoresist toobtain a plurality of hard mask islands 14 within the array region ofthe memory device. The photoresist layer may then be removed byconventional techniques, such as by using an oxygen-based plasma. Inalternative embodiments, the hard mask layer 12 may be anisotropicallyetched to obtain a hard mask grid, which can generally provide similarfunctionality (namely, protecting portions of the substrate 10 that willserve to seed lateral epitaxial overgrowth) as the hard mask islands 14discussed at length below.

As shown in FIG. 1, which illustrates a cross-sectional view of aportion of the array, the silicon of the substrate 10 is thenselectively etched back. The etch process selectively etches thesubstrate 10 relative to the material forming the hard mask layer 12.For example, a selective wet etch may be used that strips siliconrelative to silicon nitride. In another embodiment, ion milling orreactive ion etching may be used. Thus, the array of the memory devicebecomes an array of silicon studs 16, preferably with one centered oneach active area region. Each of these silicon studs 16 is definedbeneath the hard mask islands 14. Meanwhile, at least a portion of theperiphery of the memory device is preferably left untouched, protectedby unpatterned regions of the hard mask 12.

In an alternative embodiment, only one silicon stud 16 need be formedfor a plurality of active areas. For example, one silicon stud 16 may beformed for every five active areas. However, in such an embodiment, someof the steps below, such as the lateral epitaxial growth of siliconshown in FIG. 3, may take much longer. As will be understood by theskilled artisan, steps not shown in the figures are typically performedto separate the active areas. For example, in one embodiment, a fieldoxide could be defined surrounding each active area to preventinterference between adjacent active areas. Separate field isolationsteps, on the other hand, may be omitted where localized or pseudo-SOIis formed individually for each active area, as illustrated.

In FIG. 2, the portions of the array stripped according to the stepsabove are shown filled by an insulator layer 18, preferably an oxide. Ina preferred embodiment, the insulator layer 18 is blanket deposited overthe array at least up to the height of the top surface of the siliconstuds 16. After deposition of a sufficient amount of insulator, theexcess that may have formed over the islands 14 and other portions ofthe device may be removed by any of a number of processes well-known tothose of skill in the art. For example, the surface of the device may beplanarized to the top surface of the hard mask islands 14, as shown inFIG. 2. Any suitable planarization process, such as, for example,chemical mechanical polishing (“CMP”), may be used.

Thus, the array preferably comprises a plurality of silicon studs 16surrounded by insulator 18, whereas the periphery will simply remain inits original configuration with a hard mask layer 12 overlying thedielectric layer (e.g., pad oxide, not shown) covering the substrate 10.

Turning to FIG. 3, another masking process, such as that describedabove, may be used within the array to open the insulator layer 18around the silicon studs 16, at least in the regions where active areasare desired. In the illustrated embodiment, each active area has its owntrench 20, such that unetched portions of the insulator 18 betweentrenches 20 serve as field isolation. As described above, this processis preferably performed using photoresist patterned according toconventional photolithographic techniques, optionally with a hard mask.In a preferred embodiment, a selective etching process may then be usedto selectively recess the insulator layer 18 relative to the hard masklayer 12 and substrate 10, thereby forming trenches 20 in the memorydevice surrounding the silicon studs 16. This etching process ispreferably continued until the trench 20 within the insulator layer 18achieves a depth greater than the height of the hard mask layer 12 butless than the height of the silicon studs 16, thereby exposing a portionof the silicon substrate 10 forming silicon studs 16. Preferably, thetrench 20 has a depth between about 200 Å and 1,000 Å.

In a preferred embodiment, a few layers of silicon 22 may then beepitaxially grown from the silicon stud 16, using the silicon as a seedlayer. As is well-known to those of skill in the art, the epitaxialgrowth produces silicon extensions 22 with the same crystallinestructure as the silicon substrate 10. Preferably, selective epitaxy isemployed to avoid the need for subsequent removal of polysilicon fromthe exposed oxide and nitride surfaces. Preferably between about 50 521and 500 Å of silicon (or other semiconductor) is grown.

As shown in FIG. 4, a layer of amorphous silicon 24 may then bedeposited within the trench 20 formed around the silicon stud 16. In apreferred embodiment, the amorphous silicon 24 may be blanket depositedover the array, filling the trenches 20. After deposition of asufficient amount of silicon, the excess may be removed by any of anumber of processes well-known to those of skill in the art. As shown inFIG. 4, the surface of the device is preferably planarized to the topsurface of the hard mask islands 14. Any suitable planarization process,such as, for example, CMP, may be used. In another embodiment, thesilicon extensions 22 may instead be extended by epitaxial deposition inorder to fill the trenches 20. In still another arrangement,planarization may follow the crystallization step described below.

In a preferred embodiment, a thin oxide 23, shown in FIG. 4, may then begrown over the surface of the silicon layer 24, which may furtherfacilitate the crystallization of the filler silicon 24 using theepitaxially deposited silicon extension 22 as a seed layer.

The preferred silicon and oxide deposition is followed by an annealingprocess, whereby the amorphous silicon 24 has a tendency to take on acrystalline orientation similar to that of the epitaxially grown siliconextensions 22. Preferably, the amorphous silicon 24 takes an orderedcrystalline pattern. Such conversion is a species of solid phase epitaxy(SPE) known as epitaxial lateral overgrowth (ELO).

Finally, as shown in FIG. 5, the hard mask islands 14 may be removed,and the silicon layer 24 (preferably now crystallized) may be recessed.In a preferred embodiment, a selective etch may be used that etches thesilicon 24 and the hard mask layer 14 far more effectively than theinsulator layer 18, thereby exposing the silicon stud 16 for furtherprocessing steps. Further selective epitaxial deposition can then beconducted to achieve the desired thickness. Alternatively, the wholewafer can be planarized.

According to the above-described process, a localizedsilicon-over-insulator may be formed using relatively inexpensivefabrication techniques on a conventional polysilicon substrate. FIGS. 6and 7 show an arrangement where two memory cells share a singletransistor source. In particular, these figures illustrate capacitorlessDRAM formed on and within this SOI substrate. Of course, in otherembodiments, other DRAM schemes are also contemplated

In FIG. 6, a completed capacitorless DRAM structure is shown formed overlocalized SOI created according to the steps set forth above. Asillustrated, the silicon stud 16 remains beneath the common source andis connected by contacts 26 to a conductive digit or bit line 28. Thedrains 30 are located at the farthest ends of the crystallized siliconlayer 24 and are also electrically connected by contacts 32 with senselines 34. Floating bodies 36 form part of the channels that separate thedrains 30 and the source (at the top of the pillar 16) in the preferredembodiment, and these floating bodies 36 are directly adjacent an innerpair of word lines 38. This inner pair of word lines 38 preferablyseparates the source 16 and the drains 30, as may be seen in FIG. 6,serving as dual gates. While referred to hereinabove as drains 30 andsource 16, it will be understood that these are mere labels used forconvenience and for ready comparison to traditional capacitor-based DRAMdesigns. The labels can be reversed; whether the voltage is at a higherlevel at the source or the drain depends upon whether a read or a writeoperation is being performed, as described in more detail below.

The structure shown in FIG. 6 may be formed according to a number ofdeposition, pattern and etch steps well known to those of skill in theart. While configured for capacitorless DRAM operation, the illustratedscheme, whereby two memory cells share a common bit line 28 and bit linecontact 26, is otherwise similar to the scheme in U.S. Pat. No.6,660,584, issued to Tran, the disclosure of which is incorporatedherein by reference in its entirety. The '584 patent describes a “6F²”arrangement in which pairs of memory cells share a common bit line andsource region with independent pairs of word lines, drains andcapacitors. The process used to form the structure shown in FIG. 6 willdiffer, of course, from that in U.S. Pat. No. 6,660,584 to the extentthat the structure of FIG. 6 lacks capacitors.

Preferably, a gate oxide is first grown over the silicon layer, followedby a gate stack deposition and etching. The necessary doping implantsmay then be formed to define the source, drain and channel regions.Spacers 40 may be deposited and etched, in a typical spacer fabricationprocess well known to those of skill in the art, before some of thedoping steps. The bit line and cell side junctions are then formed,followed by formation of the metallic contacts and bit lines. Senseregions and other metallic contacts may then also be formed. Suchprocesses may be carried out in a number of ways, but the capacitorlessDRAM thus formed is particularly effective as a result of its formationover localized SOI. As a result of the SOI, the floating bodies 36function particularly well, isolated as they are within the insulatorlayer 18, and the devices of the periphery surrounding the array can betied to the bulk substrate 10.

In a preferred embodiment, the capacitorless DRAM shown in FIG. 6operates using gate-induced drain leakage (GIDL) current, although inother embodiments impact ionization current may also be used. As wouldbe well understood by those skilled in the art, capacitorless DRAM usesthe floating bodies 36 to store information regarding the state of thetransistor. In particular, in order to write a logical “1” value to thetransistor shown in FIG. 6, a “drain” 30 is placed at an elevatedvoltage relative to an adjacent gate (i.e., one of the word lines 38).The voltages of the drain 30 and gate 38 are controlled by the senselines 34 and the word lines 38 respectively. As a result of electrontunneling, electrons flow to the drain 30, while generated holes flow tothe floating bodies 36 underlying the gates.

As the holes accumulate in a floating body 36, the threshold voltage ofthe transistor is reduced, and the source current is thereby increased.Thus, a digital oscilloscope may be used, typically during the design ofthe capacitorless DRAM, to measure the source current and thereby thestate of the transistor. In the illustrated embodiment, this sourcecurrent may be detected along the raised bit line 28. In order to writea logical “0” value to one of the transistors, the adjacent gate takesan elevated voltage relative to the drain 30. Thus, the holes in thefloating body 36 are forced out, the threshold voltage increases again,and the source current is reduced. Again, a digital oscilloscope may beused to detect this change in source current in determining appropriateoperational thresholds. More information regarding how suchcapacitorless DRAM functions may be found in the article cited andincorporated above written by Eijiag Yoshida and Tetsu Tanaka.

As illustrated, each active area of the capacitorless DRAM forms part ofa pair of memory cells comprising two floating bodies 36, and atransistor having a single source 16 shared by the memory cells, twogates and two drains 30. The pair of memory cells, therefore, has twoaddressable locations, the floating bodies 36, that can each store onebit of data. This preferred embodiment functions generally as describedabove. However, in one application, the pair of memory cells may provideredundancy because, if either of the floating bodies 36 is storing a “1”bit, the source current at the bit line 28 is elevated. Thus, in oneembodiment, the read and write operations using the illustrated pair ofmemory cells will take place simultaneously to both floating bodies 36,thereby reducing errors.

Alternatively, the pair of memory cells may have three possible states.In one state, both floating bodies 36 store a “0” bit, and the sourcecurrent through the conductive line 28 is at its lowest level. In asecond state, one and only one of the floating bodies 36 stores a “1”bit, and the source current through the bit line 28 is at a higherlevel. Note that in this second state, the elevated source currentthrough the bit line 28 yields only the information that one of thefloating bodies 36 is storing a “1” bit, and does not indicate which ofthe floating bodies 36 is in this elevated state. In a third state, bothof the floating bodies 36 store a “1” bit, and the source currentthrough the bit line 28 is at its highest level. Thus, a sensitiveoscilloscope, for example, will be able to differentiate between thesethree states.

A schematic plan view of this capacitorless DRAM is shown in FIG. 7. Ofcourse, this capacitorless DRAM design is shown by way of example only,and the localized SOI method described above with reference to FIGS. 1-5may be used in any number of semiconductor environments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the invention. Indeed, the novel methodsand devices described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and devices described herein may be made withoutdeparting from the spirit of the invention. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the invention.

1-12. (canceled)
 13. A memory device, comprising: a source and a drain;a floating body formed between the source and the drain, the floatingbody defined within a localized silicon-over-insulator; and a gateadjacent the floating body.
 14. The memory device of claim 13, whereinthe floating body is configured to attain a first state when the drainis placed at a first elevated voltage relative to the gate.
 15. Thememory device of claim 14, wherein the floating body is configured toattain a second state when the gate is placed at a second elevatedvoltage relative to the drain.
 16. The memory device of claim 15,wherein the first state causes an increased source current at thesource, and the second state causes a decreased source current at thesource.
 17. The memory device of claim 13, wherein a stored bit of thefloating body is read by a current measured at the source.
 18. Anintegrated circuit, comprising: a periphery region; a memory arrayregion; at least one localized silicon-over-insulator formed within thememory array region; a source and a drain formed within the memory arrayregion; a floating body formed between the source and the drain withinthe at least one localized silicon-over-insulator; and a gate adjacentthe floating body.
 19. The integrated circuit of claim 18, wherein atleast a portion of the periphery region lacks a silicon-over-insulatorregion.
 20. The integrated circuit of claim 18, wherein the floatingbody is configured to attain a first state when the drain is placed at afirst elevated voltage relative to the gate.
 21. The integrated circuitof claim 20, wherein the floating body is configured to attain a secondstate when the gate is placed at a second elevated voltage relative tothe drain.
 22. The integrated circuit of claim 18, wherein a sourcecurrent is indicative of information stored at the floating body. 23.The integrated circuit of claim 18, wherein the floating body operatesby gate-induced drain leakage current.
 24. A system, comprising: asource; a first drain and a second drain; a first floating body formedbetween the source and the first drain, and a second floating bodyformed between the source and the second drain, the floating bodiesdefined within a localized silicon-over-insulator; and a first gateadjacent the first floating body, and a second gate adjacent the secondfloating body.
 25. The system of claim 24, wherein the source isconfigured to conduct a current having values indicative of threestates, the three states comprising: a first state, in which the firstand second floating bodies store a “0” bit; a second state, in which oneof the first and second floating bodies stores a “0” bit, and the otherof the first and second floating bodies stores a “1” bit; and a thirdstate, in which the first and second floating bodies store a “1” bit.26. The system of claim 25, wherein the second and third states are notdifferentiated upon detection of the current.
 27. The system of claim24, wherein the first and second floating bodies are redundant.
 28. Amethod of operating a capacitorless DRAM, the method comprising: placinga floating body in a first state; and detecting the first state bymeasuring a first current at a source of the capacitorless DRAM; whereinthe floating body is defined within a localized silicon-over-insulator.29. The method of claim 28, wherein placing the floating body in thefirst state comprises: elevating a drain voltage at a drain relative toa gate voltage at a gate.
 30. The method of claim 29, furthercomprising: placing the floating body in a second state; and detectingthe second state by measuring a second current at the source.
 31. Themethod of claim 30, wherein placing the floating body in the secondstate comprises: elevating the gate voltage relative to the drainvoltage.
 32. The method of claim 31, wherein the first current isgreater than the second current.
 33. The method of claim 30, wherein thefirst state represents a “1” bit, and the second state represents a “0”bit.